1. Field of the Invention
This invention relates to a photocationically curable composition and a process for producing the same. The photocationically curable composition according to the present invention has an excellent photo-curability and form a coating film of high hardness, so that it is useful as hard coating agents. It is also useful as a material for pollution-free paints and scribbling-proofing coating agents, photo-molding materials and photo-resist agents, when combinedly used with a silicone.
2. Description of the Prior Art
In the fields of UV-initiated polymerizations and curings, photo-initiated radical polymerizations using multifunctional acrylates and unsaturated polyesters have been extensively investigated and utilized in a commercial scale.
Such radical polymerizations are, however, not free from difficulties in that the polymerization is inhibited by oxygen, for example, in air. Particularly, curing has to be effected under an inert atmosphere in order to effect rapid and complete curing of the composition. When a coating agent composition is cured through a radical polymerization, the thinner thickness of a coating film of the composition is, the greater the polymerization inhibition by oxygen is. On the other hand, photo-initiated cationic polymerization is possible to effect the polymerization even in air, because the polymerization is not inhibited by oxygen, unlike the photo-initiated radical polymerization.
Typical monomers used for the photo-initiated cationic polymerization include epoxides and vinyl ethers. When a composition containing epoxide as the monomer is used, a cured product has an excellent heat resistance, adhesive force and chemical resistance. Particularly, when a composition containing a silicone compound having an epoxide or oxetane group is used as the monomer, a cured product has an excellent heat resistance, chemical resistance and adhesive force, as well as a good pollution-free property. Monomers having an oxetanyl group introduced at the both terminals of a silicone are mentioned in JP 6-16804 A. The present applicant filed a patent application in respect to a process for producing compounds having an oxetanyl group introduced at the both terminals of a silicone (JP Application 9-140984).
Meanwhile, silsesquioxane represented by the formula, (RSiO.sub.3/2).sub.n, is a generic term of a series of network structured polysiloxanes having a ladder, cage or three-dimensional knitting (random) structure. They are easily handled due to their solubility in usual organic solvents, unlike silica which is an inorganic material. They are featured in that they have excellent processability and formability to coating films.
As for the silsesquioxane compounds having an epoxy group introduced, hydrolyzates of (3-glycidoxy-propyl)trimethoxysilane or 2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane are mentioned (Chemical Reviews, 1995, Vol. 95, No. 5.).